This invention relates to methods of making micromechanical devices from single crystal semiconductor substrates and monolithic sensors formed thereby.
The advantages of using single crystal semiconductors such as Si as a mechanical material have long been recognized. For example, its strength and high intrinsic quality factor make it attractive for micromechanical resonant devices. It is readily available as an integrated circuit (IC) substrate and can be processed using methods developed by the IC industry.
Thick Si devices also can have advantages over thinner ones for many applications. When capacitive transduction is used to drive or sense motion in a micromechanical device, large capacitive plates with small gaps between them are desired to increase capacitance so that high sensitivity can be achieved. For laterally resonant devices, this translates into a thick structure. Thick structures can also be advantageous for inertial sensing applications where large masses are required to respond to small inertial forces.
One of the obstacles in the production of single crystal Si micromechanical devices has been the ability to integrate electrical circuitry with the micromechanics using a simple fabrication process. Integration of circuitry with micromechanics can provide a number of advantages for many sensing and signal processing applications. Often the output of micromechanical devices is a very small electrical signal. The difficulty in reading out a small output signal can limit the sensitivity or signal-to-noise ratio in many devices. This signal is usually buffered or amplified so that it can then be processed by the rest of the electronic system. When the signal processing is done on a separate chip from the micromechanics, the signal must travel through bond pads, bond wires, and external packaging structures which have large parasitic capacitances associated with them. This further limits the signal which can be read out. However, if the signal processing circuitry can be included on the same chip as the micromechanical structure, smaller signals can be amplified and conditioned, and even converted to digital signals so that when they are passed off chip, they are not degraded significantly by off-chip parasitics.
A number of technologies have been developed which integrate micromechanics and electrical circuitry. Many use surface micromachined polycrystalline Si micromechanical elements due to this material""s availability, often used as transistor gates, in electrical circuitry. The use of thin polycrystalline Si layers provides flexibility in geometry, transducer axis selection, anchoring, and number of structural layers.
However, the use of polycrystalline Si brings with it some limitations in design. Polycrystalline Si is typically deposited at relatively high temperatures. As such, when it cools down to room temperature, stress gradients developed in the film due to mismatches in thermal expansion coefficient can cause a released micromechanical device to bend. Therefore, deposition conditions must be carefully selected and monitored in order to produce a polycrystalline film with low stress. Also, film thicknesses are typically limited to a few micrometers due to the long depositions times. However, recent work has demonstrated that thick polycrystalline Si films can be grown in acceptable times with useful properties.
There are various processes used to fabricate single-crystal Si micromechanical devices but most are difficult to integrate with conventional circuit processes, or expensive Si on insulator (SOI) starting wafers are required. There have been a number of successful efforts to integrate single crystal Si micromechanical structures with conventional circuitry and all have several advantages and drawbacks.
An object of the present invention is to provide a method of making a micromechanical device from a single crystal semiconductor substrate using frontside release diffusion-etch or etch-diffusion and a monolithic sensor formed thereby without the need for wafer bonding.
Another object of the present invention is to provide a method of making a micromechanical device from a single crystal semiconductor substrate which device is integrated with conventional semiconductor circuitry with the addition of only a single standard masking step. The device may be a monolithic sensor such as a single crystal Si laterally resonant device.
This process has many advantages over other processes including the use of standard Si starting wafers as opposed to expensive silicon on insulator or exotic starting wafers. Also, this process requires no wafer bonding, no backside processing or backside alignment of the micromechanical structure, and no electrochemical etch stop for the release etch in ethylenediamine pyrocatechol (EDP).
In carrying out the above objects and other objects of the present invention, a method is provided of making a micromechanical device from a single crystal semiconductor substrate. The method includes the steps of: a) introducing a dopant into a portion of the substrate from the first surface; and b) selectively removing unwanted substrate material from a first surface of the substrate. At least one doped mechanical structure is formed after performing steps a) and b). The method further includes selectively removing undoped substrate material from the first surface of the substrate to release the at least one doped mechanical structure. The at least one doped mechanical structure is movably supported by but electrically isolated from the substrate.
The at least one doped mechanical structure may be a resonator.
Preferably, the method further includes the step of forming circuitry on the substrate.
Still, preferably, the circuitry includes bipolar circuit elements and/or MOS circuit elements.
Further in carrying out the above objects and other objects of the present invention, a method of making a suspended microstructure from a single crystal semiconductor substrate is provided. The method includes the steps of: a) introducing a dopant into a region in the substrate; b) depositing a masking material over the region; c) selectively removing the portions of the masking material to form a mask; d) selectively removing sections of the region using dry etching and the mask, wherein a doped microstructure is formed after performing steps a) through d); e) selectively removing material from the substrate with an etch which etches the substrate but does not etch the doped microstructure, so that the doped microstructure is movably supported by but electrically isolated from the substrate; and f) removing the masking material.
Step f) may be performed before step e).
Steps b) through d) may be performed before step a).
The mask may be a metal mask, a silicon dioxide mask, a photoresist mask or other mask.
The diffused doped region may be greater than 7 xcexcm thick so that the microstructure is also greater than 7 xcexcm thick or the diffused doped region may be less than 7 xcexcm thick so that the microstructure is also less than 7 xcexcm thick.
The step of introducing the dopant forms an electrically isolating diode between the doped microstructure and the substrate. Alternatively, the microstructure may be isolated from the substrate by: g) dry etching a trench around anchors of the doped microstructure; and h) refilling the trench with an insulating material such as silicon dioxide film to anchor the doped microstructure to and electrically isolate the doped microstructure from the substrate as illustrated in FIGS. 9a through 9d. 
The length of the releasing etch of step e) is variable in order to increase or decrease the distance between the suspended doped microstructure and the substrate.
The dry etching of step d) may be done in an electron cyclotron resonance source, an inductively coupled plasma source or other plasma sources.
The etch of step e) may be done using ethylenediamine pyrocatechol, potassium hydroxide, TMAH (tetramethyl ammonium hydroxide), or others.
Still further in carrying out the above objects and other objects of the present invention, a monolithic sensor is provided. The sensor includes a single crystal substrate, a doped microstructure movably supported by but electrically isolated from the substrate for sensing a condition. The sensor also includes circuitry formed on the substrate and coupled to the doped microstructure for resolving a signal based on the sensed condition.
The circuitry may include bipolar circuit elements and/or MOS circuit elements.
The doped microstructure may be fabricated by any of the above methods with the circuitry formed after step a) and before step b).
Yet still further in carrying out the above objects and other objects of the present invention, a method of making a suspended microstructure from a single crystal semiconductor substrate is provided. The method includes: a) selectively removing unwanted substrate material from a first surface of the substrate, and b) introducing a dopant into a portion of the substrate from the first surface. At least one doped mechanical structure and an open area surrounding the mechanical structure are formed after performing steps a) and b). The method further includes: c) refilling the open area surrounding the mechanical structure with a material; d) forming semiconductor circuitry on the substrate; and e) removing the material surrounding the mechanical structure. The at least one doped mechanical structure is movably supported by but electrically isolated from the substrate.
A merged circuit and micromechanics process of the invention allows integration of micromachined thick single crystal Si structures with a conventional circuit process with the addition of only a single mask. This provides the benefit of using on-chip signal conditioning circuitry to perform a variety of functions in combination with mechanics, without having to move the signal off-chip where its integrity can be compromised due to parasitics. This process could be used for a variety of devices including high performance, inexpensive accelerometers, gyroscopes, signal processing elements and various other types of sensors.
The use of single crystal Si provides excellent mechanical properties. Since the material is a single crystal, intrinsic stress in the film is minimized as compared to polycrystalline Si which is normally used as a structural material. Also, the high intrinsic material quality factor allow excellent mechanical devices to be fabricated.
This process allows single crystal Si mechanical structures with thicknesses up to 15 xcexcm or more to be fabricated. These thick structures can provide a large mass which can increase sensitivity to inertial forces. Also, this increased thickness can provide increased area for capacitive plates used for driving and sensing motion of the mechanical structure. This mechanical structure is anchored directly to the Si substrate with a continuous material and therefore, energy loss due to dissipation of energy in anchors is negligible and high quality factors can be obtained. In addition, the quality factor of laterally resonant structures are typically limited by Couette or Stoke""s type damping which is decreased by increasing the gap between the structure and the substrate. Since this gap can be arbitrarily varied in this process, and can be made large (greater than 50 xcexcm compared to typical gaps of 1-2 xcexcm on standard processes) this type of damping is minimized and high quality resonators can be fabricated.
Since this process adds only a single masking step to a conventional circuit process, devices can be fabricated inexpensively. Conventional circuit processes are very well developed and complex circuitry can be fabricated with high yields at very low cost. Typical yield problems associated with micromechanics are also minimized with this process. Since the structures can be made thick, they can be designed to be very stiff in the direction out of the plane of the chip. This makes them immune to stiction in the vertical direction which can cause failure in typical micromechanical devices. In addition, the separation between the device and the substrate is large, further preventing stiction to the substrate. Because of this resistance to stiction, standard wet release processing can be used and drying at the critical point or bridge structures are not required for successful release. Also, the thick devices are more robust than thinner devices and therefore are not as easily damaged during processing. This resiliency to typical failure mechanisms allow devices to be fabricated with high yields which reduces cost in the batch fabrication process inherent to the integrated circuit industry.
Devices have been fabricated using this process and verification of the operation of mechanical as well as electrical devices has been performed. Clamped-clamped beam comb drive resonators have been tested and excited at their resonant frequency. In addition, the transistor current-voltage (I-V) characteristics of metal oxide field effect transistors (MOSFETs) have been measured on the same chip as the resonant mechanical device.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.